If you have a long text and it is corrupted, it’s really hard to tell, particularly before the invention of checksums. Is this a hapax legomenon or did someone just misspell a word? Even things that should be clear like “is this part of the text or part of the commentary on the text” get murky: there are several parts of the Bible where scholars will say things like, we think this was meant to be an explanatory comment but it got put in the main text by mistake.

Oral tradition can make these problems better because if one person has a distorted view of the text, they won’t be able to be certified as a reciter by their master. The Indians in particular had various checksum methods, plus a tradition of teaching how to pronounce things, which all would have made the text drift much slower than “a scribe messed this up and the only way to tell is to go to another region and see if their copy has the same words or not.”